Prostanthera denticulata

Rough Mint Bush

Family: Lamiaceae

A wiry / weak-wooded, semi-erect to prostrate shrub, aromatic, growing to a height of 1 metre and potentially spreading to 2 metres wide.

It is found growing in damp areas in sclerophyll forest and woodland, usually near the coast, in sandy loamy soils, overlying sandstone.

It extends from the Northern Tablelands of NSW and the Central Coast of New South Wales. There are also records much further west in central NSW as well as in Victoria; however, these specimens appear to belong to a different taxon.

A view is now held that P. denticulata (in the strict sense) is confined to the Sydney district.

Stems are square with rough hairs.

Leaves are in opposite pairs or rarely in 3-leaf whorls, egg-shaped, to 10 mm long and to 3 mm wide on a petiole to 1 mm long.

Flowers have a shape described as labiate (applies to all Lamiaceae flowers) with 5 petals varying in their size, fused at their base, produced in leaf axils. One of the identification features for Prostanthera is that the 5 calyx parts (basal whorl of the flower) are fused into 2 lips.

In this species, flowers are arranged in leaf axils near the ends of branchlets with attached bracteoles to 1.5 mm long at the base.

The petals are fused into a tube, to 10 mm long and wide, purple to mauve. Flowering occurs in spring and early summer

Fruits – 4 tiny nutlets (mericarps) produced at the base of the calyx.

In the garden

This species is sold online from several outlets. It prefers a well-draining soil with morning sun and afternoon shade, so try to provide a southern to eastern aspect.

Needs some supplementary watering in dry times.

Can be pruned to shape – give a light tip prune after flowering to encourage a denser shrub. Could be tried in a large pot as well.

A Prostanthera in full bloom is a magnificent sight and there are so many colours to choose from for your garden. These plants are found in all states in varied soil conditions and climate and thus while it may be a challenge to grow some species many are easy in a garden situation.

A few basic growing tips are:
• Good drainage is essential. Raised beds ensure this
• Water new plants until established, weekly or as required.
• Do not over water, as this can induce root rot and fungal infestation.
• They prefer moist root runs.
• Plant drooping is an indicator of dryness

Propagation

Plants may be grown from fresh seed. However, cuttings are frequently and reliably used, usually semi-hard wood or soft tip material, which strike well in spring or autumn.

Other information

There is a cultivar sold online called ‘Purple Haze’ which is a form of this plant.

There are approximately 100 species, endemic to Australia. They occur in all States.

This genus is currently under revision, and several species complexes are unresolved. Natural hybrids occur between several species and most species appear to be capable of hybridizing when in cultivation.

Some 80% of mints contain aromatic oils within their leaves with oil of cineol being a major component. Prostanthera sieberiP. incisa and P. staurophylla are quite pleasantly overpowering due to their exudates when crushed.

Positioning of prostantheras as border plants or near pathways is recommended as the mint odour is released when brushed against. Oil from the leaves of some species is distilled for use in cosmetics and as soap additives.

Prostanthera – from the Greek prosthike (προσθήκη) which translates to “addendum”, and anthir (ανθήρ) meaning anther – referring to the anthers which have an appendage of tissue.

denticulata – is a Latin word meaning “with small teeth” referring to the edges of the leaves.

https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=gn&name=Prostanthera
http://anpsa.org.au/APOL2007/apr07-1.html
https://en.wikipedia.org/wiki/Prostanthera_denticulata
https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Prostanthera~denticulata

By Jeff Howes